Medical solutions like dialysis solutions for hemodialysis, hemofiltration, hemodiafiltration, peritoneal dialysis, dialysis within renal intensive care and liquids for substitution or infusion normally contain a buffering substance. Often used buffers are acetate and lactate buffers and these buffers are within the human body metabolized into bicarbonate. Thus, the most physiological buffer in medical solutions would be bicarbonate.
However, the use of bicarbonate as a buffer is more complicated than the use of acetate and lactate for two reasons: First bicarbonate easily precipitates with one of the essential elements in dialysis fluids, viz. calcium, to form calcium carbonate, and second bicarbonate solutions emit carbon dioxide and are thus unstable.
One way to get around the precipitation problem is to separate bicarbonate and calcium in two different containers and then mix them just before use, but the problem with the emitted carbon dioxide still remains.
If carbon dioxide leaves the bicarbonate solution the result is an increase of pH up to 9-10.5 depending on the original bicarbonate concentration. According to prior art, this problem is solved either by use of a gas barrier for carbon dioxide or by allowing the bicarbonate to slowly equilibrate with the atmosphere.
If a gas barrier is used, a complicated and expensive polymer is required as gas barrier otherwise it will result, after mixing with the rest of the content in the container, in an non-definable pH (depending of age of the solution). The polymers used for these types of gas barriers are often very brittle, and care has to be taken when handling and storing the bags with gas barriers not to create cracks that will give rise to leakage.
The idea of letting the bicarbonate slowly equilibrate with the atmosphere is for example disclosed in U.S. Pat. No. 6,309,673. However, this way of solving the problem creates an uncertainty concerning the pH value and the bicarbonate concentration in the final, ready-to-use solution.
In U.S. Pat. No. 5,296,242 a solution is disclosed in which a premix of bicarbonate and carbonate is used in the buffer system. This document discloses a specific mix of bicarbonate and carbonate, which provides for a partial pressure of carbon dioxide that equals the physiological value of the partial pressure within the human body. The buffer solution is further combined with an acid solution, the acid being a metabolizable, organic acid. This reference stresses that an organic acid should be used, this for the therapy of acidosis.